Providers::NginxSite: set of default matchers

Use it this way:

**recipe**

```ruby
nginx_site "groupon-application" do
  template "vhost.erb"
  variables(root: '/home/groupon/groupon/current/public')
  action :create
end
```

**spec**

```ruby
it { is_expected.to add_nginx_site 'groupon-application' }
```

or

```ruby
it "should add nginx site 'groupon-application'" do
  expect(subject).to add_nginx_site 'groupon-application'
end
```